Clouds are made of very small drops of water of water, ice crystals, and other small particles in the atmosphere (mass of air surrounding Earth). The water comes from condensation, a process that allows small drops of water to form as the air cools. Cloud shapes and the way clouds form give scientists important clues about local weather and conditions in the atmosphere around the world.

Clouds are divided into several types or families of clouds. These families of clouds are named according to where or how they form, and include high-level clouds, middle-level clouds, and low-level clouds.

In addition to belonging to a family, clouds are also named for their shape. Puffy clouds are known as cumuliform clouds, and flat sheet-like clouds are known as stratoform clouds.

How clouds form

In general, as warm, moist air rises upward through the atmosphere, the air cools. As the air cools, ice crystals or water drops appear and clouds form. Meteorologists (scientists who study weather and climate) name clouds based on how they form, where they form, and the shape of clouds. Cloud classifications are organized into groups or families.

Families and types of clouds

The altitudes (heights above the ground) used to describe cloud families change and become lower as one moves from the equator toward the North or South pole. As one moves north or south from Earth’s equator (imaginary circle around Earth between the North and South Pole), high altitude family clouds can be observed at much lower altitudes.

High level clouds include cirrus, cirrostratus, and cirrocumulus clouds. These clouds are found at altitudes between 16,000 and 45,000 feet (4,877 and 13,716 meters) above the ground. In comparison, a jumbo passenger jet usually cruises at about 36,000 feet (10,973 meters) above the ground.

cirrus clouds

Middle level clouds include altostratus, altocumulus, and nimbostratus clouds, and are found between 6,500 and 22,000 feet (1,981 and 6,706 meters) above the ground.

These clouds include include altostratus, altocumulus, and nimbostratus clouds. As with many cloud families, the altitudes are not exact, and they can vary depending on the type of terrain (sea or mountains) over which the clouds form or travel.

Low-level clouds include stratus and stratocumulus clouds that are found below 6,000 feet (1,829 meters).

Clouds that form rapidly in a vertical (up and down) direction are known as vertical development clouds. Vertical development clouds include cumulus and cumulonimbus clouds, which are the clouds that form a thunderstorm.

Vertical development clouds form rapidly as air rises from the Earth’s surface. They are found anywhere from the surface of the ground to 45,000 feet (13,716 meters).

In some very strong thunderstorms, the clouds may reach even higher. One factor that contributes to the development of thunderstorms is unstable warm and humid air that quickly rises through the atmosphere to great heights where the surrounding air is very cold.

Shape and color of clouds

The shape of a cloud is determined by the manner in which the water drops condense and the forces of winds that can act to tear away pieces of the cloud as it builds and moves in the atmosphere.

Whether a cloud is light or dark depends upon how much light can pass through the cloud. Water droplets bend or block light. Thicker clouds block more light than thinner clouds, and so appear darker than thinner clouds.Names of clouds

When clouds are widely separated from other members of their family, the term fracto is added to their name. When a cloud produces rain (precipitates) it is also called a nimbus cloud and the term nimbus is added to the cloud name.

High clouds

Cirrus clouds occur at high levels and are usually wispy and long.

If air rises directly upward through the atmosphere, the air cools very quickly. As the air cools, ice crystals or water drops appear and cumulus clouds form. Cumulus clouds are billowy, puffy clouds that resemble cotton balls.

Stratus clouds

Stratus clouds look as if they are blankets or layers of clouds.

Because it is very cold at high altitudes, high clouds including cirrus clouds, cirrostratus clouds, and cirrocumulus clouds are composed of ice crystals. Particles of dust or pollution often form the center around which the ice crystals grow.

For this reason, dust or particles of pollution are often called centers of crystallization if ice grows around them, or condensation nuclei (nuclei meaning the center) if water drops form around them.

Cirrus clouds often produce a shape that looks like a horse tail. These “mares’ tails” are wisps of ice crystals. Cirrostratus clouds, because they are thin and because their ice crystals act to both reflect and bend sunlight, sometime appear to form a circle or halo around the Sun or Moon.

Cirrocumulus clouds often appear as patch-like thin clouds.

Middle level clouds

Middle level clouds include altostratus clouds, altocumulus clouds, and nimbostratus clouds. These clouds are composed of water drops with some ice crystals near the top of the clouds.

altocumulus clouds

Sometimes both middle level and low level clouds contain water that is still in liquid water drops even though the air around them is well below the freezing temperature. This super-cooled water (water below freezing that has not yet formed an ice crystal) needs only a seed, usually a particle of dust or pollution around which to form ice.

At one time, scientists experimented with making rain by seeding clouds with a chemical called silver iodide. It was hoped that the silver iodide would provide a center around which large water droplets would form. When the water droplets grew large enough, they would fall as rain.

Cloud seeding thus offered hope that it might be possible to produce rain in dry regions. The results of these early experiments were disappointing, however, and produced little rain beyond the amounts that fell without cloud seeding.

Because of the way ice crystals reflect and deflect light, altostratus clouds often present a bluish-layered appearance. Depending on thickness, altocumulus clouds often have white or gray layers that appear in washboard or wave-like formations.

Warm moist air that rises can also result in the formation of castle-like altocumulus castellanus clouds, a form of altocumulus that often appear as isolated cumulous clouds with billowing tops.

Another form of altocumulus cloud is called a standing cloud (properly termed a lenticular altocumulus cloud), and is formed by condensation in currents of air that cool as they move upwards to cross mountains and ridges.

Although constantly forming and disappearing, the standing lenticular altocumulus cloud formations appear not to change and thus seem to stand over the mountain or ridge lifting the air.

Clouds that pass through many levels of the atmosphere, the cumulus and cumulonimbus clouds, often have a widely varying mixture of ice and water. These clouds often have swirling currents of air that move upwards and downwards.

These rapid updrafts and downdrafts of air allow ice crystals to appear at much lower levels than normal. As they cycle through the cloud, the ice crystals can grow large enough to fall to the ground as hail.

cumulus clouds

Although formed from air rising upward from the ground and lower levels of the atmosphere, cumulus clouds often form in fair weather and do not form violent updraft or downdraft currents of air. These cumulus clouds have flat bases and curved tops that look like domes of buildings.

When strong and violent updrafts and downdrafts of air form, however, the air is said to be unstable and the cumulus clouds are said to be more developed. These cumulus clouds have mushroom or cauliflower-like tops, and they often produce rain.

When cycles of air moving upwards and downwards become very violent, cumulonimbus clouds form. Cumulonimbus clouds are dark clouds with anvil-like tops (very flat tops with trailing clouds spreading out like a tabletop) that are often cut off (sheared) by strong winds in the upper atmosphere.

Thick cumulonimbus builds on the plains of red barn.

Cumulonimbus clouds often have heavy turbulence (rough and violent disturbances of air), rains, lightning, and thunder. The most unstable and violent cumulonimbus clouds can occur in cells or groups capable of forming tornadoes.